To allow for people having speech and/or hearing disabilities that prevent them from using conventional telephones to communicate over the public switched telephony network, text telephones (TTY devices), also known as telecommunications devices for the deaf (TDD devices) have been developed. In general, such devices encode characters of text using sequences of audible tones. In particular, in response to receiving a command to transmit a character, a TTY device will generate a sequence of audible tones that is transmitted through the telephone network to a similar TTY device at the receiving end. The TTY device at the receiving end decodes the sequence of audible tones, and displays or otherwise outputs the encoded character.
Text telephone devices operate according to various operating protocols or standards. When a call is placed between countries or regions adhering to different protocols, standard TTY devices become inoperable. For example, in the United States, TTY devices communicate with one another using a 45.45 Baud frequency shift key protocol commonly referred to as Baudot signaling. Baudot signaling transmits characters using a sequence of seven audible tones at either 1400 Hz or 1800 Hz. In particular, a Baudot character comprises a start bit of 1800 Hz, five tones of either 1400 or 1800 Hz to signal the series of five bits specifying the character, and a stop bit of 1400 Hz. There is no error correction. There is no “handshake” tone, nor is there a carrier tone. (TTY devices that use this style of encoding are silent when not transmitting.) At 45.45 Baud, the duration of each individual tone signaling the start tone and the five tones specifying the character is 22 milliseconds. The stop tone is often 33 milliseconds in duration, and can be 44 milliseconds.
The standard for TTY devices that is commonly employed in the United Kingdom, Ireland, Australia, and South Africa is identical to the standard used in the United States, except that the system is 50 Baud. Accordingly, tones that in the U.S. TTY protocol are 22 ms in duration are 20 ms in duration according to the U.K. protocol. This difference is sufficient to prevent devices designed to operate using one of the protocols to interoperate with devices designed to operate using the other protocol.
For a variety of reasons, particularly problems caused by packet loss and audio compression, it is not practical to use voice or audio channels on packet-switched Voice over Internet Protocol or VoIP wide area networks to transmit TTY tones. For this reason, various non-audio data protocols, such as RFC2833 and RFC2793, are being used to convert the TTY tones into equivalent packet payloads for transmission over the Internet.
Because many TTY callers currently do not have the necessary software to make Internet calls, most TTY calls are still made over traditional circuit-switched networks, such as the Public Switched Telecommunications Network or PSTN. This includes traditional TTY calls that may be directed to a resource that is on an IP or VoIP network, thereby often necessitating a hybrid architecture in which a gateway device translates between audio TTY tones on the circuit-switched network and equivalent non-audio data packets on the packet-switched network.
Even though the problem of reliable transmission of TTY signals on VoIP networks has been addressed in the prior art, a problem that remains is that call centers, including 9-1-1 emergency services and 2-1-1/3-1-1 information services, presently lack an inexpensive mechanism for identifying incoming PSTN calls that are from TTY users (hereinafter referred to as TTY-enabled calls). As a result, the routing of those calls to an appropriate TTY-enabled resource has tended to be inefficient. There are a number of reasons for this deficiency. For example, unlike facsimile machines and computer modems which transmit CNG tones, TTY's do not emit a self-identifying handshake tone or a carrier tone. Even though some call center configurations prompt TTY callers to enter a specified DTMF touch tone command to identify themselves as TTY callers, many TTY's, including the Avaya™ Model 8840™, are unable to generate DTMF signals after the call is placed. (Such TTY's switch automatically to TTY mode after generating the necessary DTMF signals to dial the call.) As a result, many TTY callers are unable to make menu selections with traditional DTMF-input automated attendant and IVR systems.
Because many TTY users are unable to transmit DTMF signals, IVR systems that are able to accept TTY-format menu selections, such as the Avaya Interactive Response™ platform, have been developed. However, even with these systems, a complicating factor is that the Baudot communications protocol used by TTY's is moded, i.e., the same sequence of tones can actually correspond to different characters. (Illustratively, the five-bit sequence 00001 can correspond to the letter E or to the number 3, depending on whether the TTY is in letters mode or numbers mode. Because the protocol itself is half-duplex, the user of a TTY device who has tried to transmit the letter E has no way of knowing whether the receiving device decoded this transmission as an E or as a 3.) To overcome the complications of moded output and make menu selections possible, specialized software—unnecessary in DTMF-only systems—is often required to resolve the potential ambiguity of the input. Many call centers use such specialized software to provide automated attendant and IVR systems that can accept TTY-format inputs in place of DTMF input. However, in call centers in which the only application for the software would be to vector a TTY call to a TTY-equipped agent, the use of such TTY-decoding software represents a very expensive solution.
As a consequence, most call centers that desire to be directly accessible to TTY users provide a separate phone number for TTY callers. Unfortunately, provision of separate voice and TTY inbound telephone numbers represents an additional expense for these call centers, especially if both of the inbound lines are “toll free.” Although this practice is currently used by many private call centers, it is now barred by the U.S. Department of Justice for 9-1-1 services, chiefly because, in a typical installation, it would not be possible to provide a separate easy-to-remember three-digit number for the TTY users. The configuration recommended by the U.S. Department of Justice for 9-1-1 call centers is to equip all agents with TTY devices, an approach that adds significantly to the equipment costs for 9-1-1 centers.